Field of Invention
This invention relates generally to light-weight cores of high strength for inclusion in laminated structures, and more particularly to a contourable core formed of a planar array of block-like modules made of end-grain balsa wood or other material whose adjacent edges are held together at their adjacent edges by flexible adhesive joints, and to a technique for fabricating such cores.
A technique for fabricating cores in accordance with the invention, though applicable to various species of wood, is of particular value in connection with balsa wood derived from a tropical American tree (Ochroma pyramida). Balsa wood has outstanding properties unique in the lumber field; for on the average, it weighs less than 9 pounds per cubic foot, this being 40% less than the lightest North American species. Its cell structure affords a combination of high rigidity and compressive and tensile strength superior to any composite or synthetic material of equal or higher density. While a technique in accordance with the invention will be described herein only in regard to balsa wood, it is to be understood that it is also applicable to many other wood species, as well as to rigid foam plastic and other materials having acceptable properties in the context of laminated structures.
The market potential for balsa wood is considerable; for useful structural sandwich laminates can be created by bonding thin facings or skins to balsa wood panels which function as a core. Thus the Kohn et al. U.S. Pat. No. 3,325,037 and the Lippay U.S. Pat. No. 3,298,892 disclose structural sandwich laminates whose core is formed of end-grain balsa wood, the laminates having an exceptionally high strength-to-weight ratio as well as excellent thermal insulation properties.
End-grain balsa-cored sandwich laminates are widely used in transportation and handling equipment, such as for floors for railroad cars, shipping containers, cargo pallets, bulkheads, doors, reefer bodies, as well as in a wide variety of other applications. These laminates are also employed for structural insulation in aircraft applications, housing and in boating.
Where the structure to be reinforced is constituted by planar surfaces, the balsa core may simply be a solid board or panel laminated to the facings. But in the case of hulls and other structures having single or double curvatures, or other complex contours, it is ordinarily not possible to conform solid balsa to the contoured surface without bending the balsa panel, and this involves difficult, time-consuming and expensive procedures.
As noted in the Shook U.S. Pat. No. 3,540,967, contourable balsa blankets have been developed that are composed of small individual balsa blocks cut from a board. The blocks are attached to a common carrier, such as a fabric scrim, whereby the blanket may readily be conformed to a curved surface for lamination thereto.
Such blankets, which are commercially available from the Baltek Corporation of Northvale, N.J., under the trademark "Contourkore," are useful in the construction of reinforced plastic boats and larger vessels, for they lend themselves to lamination between layers of resin-reinforced fiberglass or other plastic material, thereby bringing about a distribution of weight favorable to high stability and buoyancy, as well as imparting stiffness to the structure.
As pointed out in the above-identified Kohn et al. and Lippay patents, quite apart from the structural merits of balsa, this wood is of particular value in cryogenic applications, for it has a low coefficient of expansion and hence deforms only slightly under severe temperature changes. Moreover, the k-factor of balsa wood is such as to render this material highly suitable as thermal insulation. The symbol for thermal conductivity is the k-factor, this being the amount of heat expressed in BTU's transmitted in one hour through one square foot of homogeneous material, one inch thick, for each degree of Fahrenheit of temperature difference between opposed surfaces of the material.
The above-noted Shook patent discloses a contourable blanket having an array of end-grain balsa wood blocks attached by pressure-releasable adhesive lines to a fabric scrim made of a non-woven, open-mesh material of fiberglass or other nonstretchable yarns of high tensile strength. Because of the open mesh, the surfaces of the blocks are almost fully exposed to facilitate lamination. On the other hand, the stability of the scrim maintains the balsa blocks or modules at their assigned positions in the planar array thereof and prevents overlapping thereof during handling. Because fiberglass is wettable, it may be effectively bonded by standard resins and other adhesive agents both to the blocks and to the laminating plies.
When the blanket is laid down on a contoured surface for lamination thereto with the scrim facing out, and the blocks are then pressed into conformity with the surface by a roller or other means, each block will assume an orientation determined by the surface engaged thereby. Where the orientation of a block varies from that of the adjacent block as will occur in complex curvatures, the block will then partially detach itself from the scrim to the extent necessary to assume the desired orientation. Thus for a given contoured surface, selected blocks in the blanket are detached partially from the scrim, such detachment being facilitated by the adhesive line connections which permit release when pressure is applied to the block.
In working for many years with Contourkore blankets, we have found that whereas one obtains reasonably good bonding in the interface between the scrim side of the blanket and a facing ply or sheet laminated thereto, one achieves better bonding in the interface between the naked side of the blanket and the facing sheet laminated to this side. Since the strength of the resultant laminated structure depends in good part on the strength of the bonds between the core and the facing sheets, the relative weakness of the scrim side bonding line represents a negative factor.
Another factor which militates against a contourable scrim blanket is that because the scrim is attached to the balsa wood blocks on one side thereof, it creates an unbalanced structure; for if the wood swells because of a change in its moisture content, it is restrained only on the scrim side and not on the naked side. As a consequence, Contourkore blankets are subject to warping, which makes it difficult in some instances to hold it down on a resin-coated contoured surface when this coating is in the wet, uncured state.
Yet another drawback of a scrim blanket is that because the flexible scrim is non-stretchable, the blanket is conformable to a contoured surface whose curvature extends in one direction only. But if the contoured surface has a curvature running in the longitudinal direction as well as a curvature running in the transverse direction, the scrim blanket can be bent to conformity in only one of these directions, for the scrim then resists conformity in the other direction.